edited by GEORGEL. GILBERT Denison University Granville. Ohio 43023
tefted demon/trcltion/ Collapsing Can S u s ~ l m oBY
Equipment One 12-cm crystallizing dish; 10 cm of copper (or platinum) wire; forceps; overhead projector.
Richard D. Sands Alfred University Alfred. NY 14802 CMECKEO BY
David Blackman University of the District of Columbia Washington, DC The considerable solubility of ammonia gas in water can he demonstrated hv the introduction of a small amount of water into a closed c& filled with the gas. The water dissolves the ammonia. creatina such a vacuum that the can collaoses. A gallon ran (a plastic CloroxQhottle dfers the a(ivantage that it can he reused) with a small screw-cap is dried and filled with ammonia. Enough of the gas is introduced in 30-60 see by a fairly slow stream from a cylinder of ammonia. -~ltern&vely,the gas may hegenerated hy the addition of concentrated ammonium hydroxide solution to sodium hydroxide pellets. A side-arm flask is fitted with a separatory funnel and a tube leading to the can. A teaspoonful of sodium hydroxide pellets (about 50) is placed in the flask and covered by the slow addition of the ammonium hydroxide solution (50-100 ml) from the separatory funnel. When the can has been filled with ammonia, it is sealed with a one-hole rubber stopper fitted with astopcock to which an carsyringeor largedropping bulhcan beattarhed. From the ear svrinee. throueh t h itnowck. ~ a suuirt cahout 30 mlf nf wate; is i%roducA into theean and t i e stopcock is quickly closed. Sometimes a t once. other times after a little shakine. -. the can collapses. When a can secretlv filled with ammonia ahead of time collapses upon the introduction of water, the listeners' interest will certainlv he held lone " enouah to exolain and to fill and collapse another can.
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Reduction Potentials and Hydrogen Overvoltage: An Overhead Projector Demonstration SUBMITTEDBY:
Richard W. Ramette Carleton College Northfieid. MN 55057 CHECKED 0 ~ :
Bob Olsen University of Wisconsin. Madison Madison, WI 53706 This demonstration relates the scale of standard reduction potentials to the nbserv~dbehavior of me& in their renrtions w ~ t hhydrogen ion to producr hydrogen gas. It illustrates the kinetic inhibition of hvdroeen eas evolution on a mercurv surface and provides dramatic evidence for the validity of an electron-transfer mechanism. I t provides a setting for the discussion of the difference between thermodynamic reaction tendency and kinetic tendency. ~
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886
Experimental
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Journal of Chemical Education
Reagents 100 mL 6M HC1; 0.5 g HgClz dissolved in 25 mL 1M HCI; mossy zinc Procedure Step I: Place the crystallizing dish, containing the 6M HCI and the copper wire, on the overhead projector and focus sharply on the wire. There is no visible reaction to produce hydrogen by the reaction Cu(s) + 2H+ = Cu2+ Hp(g). On the blackboard draw a vertical line representing the scale of reduction potentials from -1 to +1V, and mark it to show the SHE reference point, the Eofor Cu2+/Cu at +0.34 V, and the E o for Zn2+/Zn a t -0.76 V. Discuss the use of potentials to predict thermodynamic tendencies, pointing out the very different positions of copper and zinc relative to hydrogen. Step 2: Remove the wire and place a piece of mossy zinc in the acid. The evolution of hydrogen is in accord with the thermodynamic prediction, and the fact that i t is rapid shows that the reaction also has a strong kinetic tendency as well, that is, a mechanism with low activation energy. Remove the zinc with forceps. Step 3: At the lecture bench place another piece of zinc metal in the mercuric chloride solution and let i t stand while discussing the reduction potential of HgZ+/Hg (EO = +0.85 V). Thermodynamics predicts that mercury metal will not reduce hydrogen ion, hut that zinc metal has a very strong tendency to reduce mercuric ion to mercury metal. The surface of the immersed zinc will appear shinier, but only at close range, due to the coatine of mercurv. Remove the amaleamated zinc with forceps and rinse very thoroughly to remove traces of mercuric chloride solution. C a ~ o e dvials containine mossy zinc, and freshly-amalgamatedzinc, may he passe& around the class for inspection. The amaleamated zinc should l prevent zincoxide furmntim on be covered wirh 1 M ~ i ' to the surface hut rihndd not he stored with a tight cao because there may be a pressure buildup due to veryslow hydrogen evolution. Step 4: Place the piece of amalgamated zinc in the acid and observe that there is no rapid evolution of hydrogen. After a few minutes there will he a few huhhles clinging to the surface, but the rection is extremely slow in comparison to that of the untreated zinc. A tentative explanation (elicited from the class if possihle) is that the coatine of mercurv on the zinc surface acts as a shield, preventingihe co~~isiodof hydrogen ions with the zinc. However, zinc metal is fairly soluble in mercury. The coating is a saturated solution (amalgam) with a substantial supply of zinc atoms throughout and a t the surface itself. Therefore one would still predict, on the hasis of thermodynamics, that there should be evolution of hydrogen gas. The extremely slow evolution must he explained in terms of kinetics, not thermodynamics. Althoueh hvdroeen atoms may he formed on the surface, H30+ e - 2 ~:(honhedto the
+
+
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surface) HzO, there is a high activation energy associated with their movement across the surfaceto form molecules. The used by electrochemists for this behavior is overvoltage. Hvdroeen evolution on a mercurv surface does not ~ r o c e e d -" rapidly until the surface is made to have a potentialahout 1 V more negative than the thermodynamic prediction. Step 5: Place the copper wire in contact with the amalgamated zinc. under the surface of the acid. Hvdroeen . - evolution is rapid, h i t it occurs on the surface of the copper, not on the zinc. Explain that the zinc, which really does have a strong thermodynamic tendency to serve as a reducing agent, can release its electrons to the copper wire, thus giving it a negative potential. Since the overvoltage for hydrogen evolution on a copper surface is small, the gas evolution proceeds smoothly. The formation of Hz stops and starts as the wire is disconnected and reconnected. Meanwhile, hack a t the zinc surface, zinc ions enter the solution as required for electroneutrality. In fact, amalgamated zinc is an excellent reducing agent for many substances other than hydrogen ion and is widely-used in the Jones reductor for this purpose. I t provides a powerful
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reducing capability that can he used even in acid solutions without simultaneous hydrogen evolution.
Notes Two hooks which contain a good treatment of overvoltage phenomena are "Textbook of Electrochemistry," [G. Kort"m and J. O'M. Bockris, Vol. 11, 19511 and "Electroanalytical Chemistry," [J.J. Lingane, 2nd Ed., 19581. If both copper and platinum wire are available, the following modification is possible. First connect the copper wire to the zinc amalgam and draw attention to the increased gas evolution. Then connect the platinum wire to the copper wire (hut do not connect the platinum wire to the zinc amaleam). Gas evolution now occu& almost exclusively at the plat&un. The conclusion which can he drawn is that hvdroaen . - overvoltaee " increases in the order P t < Cu < Hg. Spattering of the acid solution may take place as the zinc dissolves. For this reason, the projector should he wiped with a damp cloth after use. Proper disposal of the mercury-containing reagents is the major precaution.
Volume 59
Number 10 October 1982
867
